(1) Field of the Invention
The present invention relates to a system for injecting liquid chemical into a subsea well and to pumps designed for use in such a system. Although the term “subsea” is used for convenience to indicate the location of wells to which the system relates, this should be understood to include reference to any substantial body of water beneath which a well may be located. Furthermore pumps of the character to be more particularly disclosed herein are not restricted to use in such systems and may also find application in, for example, automotive fuel injection systems, hydraulic actuator systems, or in other areas where high fluid pressures need to be generated by electrically-powered pumps with a minimum of moving parts.
(2) Description of the Art
It is a well known practice, in order to maintain the efficient operation of a production oil or gas well, to inject certain chemicals in liquid form into the well at selected times and positions, for example corrosion inhibitors to inhibit corrosion of downhole equipment and wax inhibitors to inhibit the formation of waxy substances that block the flow of product. For high pressure, high temperature (HPHT) wells and extremely high pressure, high temperature (XHPHT) wells, pressures typically in the range of 15,000-25,000 PSI (100-170 MPa) need to be generated by the pumps in such systems. In the case of subsea wells it is not always practical to have pumps at the surface platform (or only at the surface platform) due to the cost of running high pressure umbilicals down to the wellheads (which can involve umbilical lengths of some thousands of meters) and the pressure drop across such long umbilicals, meaning that control of the delivery pressures and flow rates at the wellheads can be quite problematic. It is therefore common to employ the pumps (or additional pumps) for such systems underwater in the vicinity of the wellheads. However, a subsea environment presents particularly serious challenges to the reliability of such chemical injection pumps due to the aggressive conditions under which they are required to operate and the difficulty of accessing and effecting any required maintenance or repair of the equipment located underwater. Current systems typically employ hydraulically-actuated pumps, requiring hydraulic control lines to be run down to the sea bed, and regular maintenance, and are therefore both complex and costly to operate. The present invention therefore aims to provide an alternative pumping system for such service, which can be electrically operated, has a minimum of moving parts and in particular avoids the need for any rotating parts and attendant high performance bearings and seals; in other words an essentially “solid state” solution.